THAT is the titilating sextant, a delight to perform with, and part of a ceremony utterly maddening to fathom.

It’s got two glass pieces: point one to the horizon, the mirror to some celestial body. Slide the arc to tilt the mirror to bring the star or planet down, or hold upside down to bring the horizon up so the two touch. Gently rock to and fro to be sure it’s true while chanting, “ready…ready…ready…MARK!” at which point an assistant makes note of the time while you read the angle off the sextant. Repeat, using different reference points unless your original celestial body was the Sun at noon sharp.

To do it the way it used to be done, take the angle of the celestial body to the horizon, go through a few mathematical calculations, and then with the numbers, you consult the Oracle, the Thick Book, or Nautical Almanac, with positions predicted thousands of years from now–with corrections–of the major celestial bodies’s paths charted mostly by the ancient Egyptians, who observed and recorded for eons, appended with over 20 years of work by Tycho Brahe, and mixed with laws of motion by Johannes Kepler.

It ain’t perfect: it is dependant upon your equipment, 22+ mathematical calculations, a moving boat, the time piece, visibility, weather, wind, currents, air temperature, time zones, atmospheric refraction, fatigue, etc etc. But if you know how to use the tools: a sextant, a timepiece and the Thick Book, you can find out where on this blue marble you are located.

Coastal merchant mariners are required by the Coast Guard to demonstrate the ability to take readings with noon sun, sunrises, sunsets, three star fixes, and running fixes (I think.) The US Naval Academy discontinued teaching it in 1998, feeling that celestial navigation did not give an accurate enough result to warrant the labor required in what was considered the most challenging course in its curriculum, preferring to rely upon computers.

Global Positioning System (owned and operated by the United States Government and stewarded by the Department of Defense), or equipment that reads and translates satellites’ signals, can go down; it’s happened.

I tried to learn it out of curiosity, for the GPS on the schooner was just that, the Grey Plastic Sextant.

Taking the readings from the Food Court terrace on Pier 17 was fun, but then I was deposited into the Abyss: the numbers took me into the Labyrinth, and I was left to wander through a mad world of numbers-sorcerery, azimuths hanging overhead, thedas lurking on the horizon, angles flopping, calculations thrashing and clashing, I was hopelessly lost…

Capt Don Chesley was my teacher, who, in college, was so enthusiastic about celestial nav that he would take readings from his dorm window using his frisbee, filled with water to reflect and reveal the horizon. He teaches it well, I have been lucky to hear him at the Seaport Museum and at Stevens Institute, and it is not a reflection on him that I do not get it.

“The scale of a sextant has a length of 1/6 turn (60°); hence the sextant’s name… An octant is a similar device with a shorter scale (⅛ turn, or 45°), whereas a quintant (1/5 turn, or 72°) and a quadrant (¼ turn, or 90°) have longer scales.” —wiki.

Lacking the ThickBookOracle, you can put your numbers taken off your sextant here.

–thank you, Capt Benjamin Dutton, J. and BigSecurityMama
sextant illustration is a simplified version from the amazing Lore of Ships by Tre Tryckare